Inside Water heater wiring

Just wondering what type of insulation jacket is required for the internal wiring for a water heater? What is the temperature rating of the insulation? Is it required to be damp location or wet location? I'm adding another element to my single element water heater and I need the appropriate wire to tie in the new element and thermostat. It appears the existing wire is rated 105 degrees Celsius, but I'm having trouble finding similar wire. Any help would be great. Thank you.